I've been doing a little digging and consequently have been able to add some more information to two of Chailey's men - Thomas Clarkson and John Mitchell.
Service numbers can tell you a lot about a soldier. The main problems are though that a) the British army did not adopt a unique numbering system and b) there was often inconsistency in recording the numbers; prefixes being omitted.
In John Mitchell's case, Chailey Parish Magazine notes that he served first with the 2/4th Royal Sussex Regiment and then with the 12th Battalion. The National Archives notes two numbers for him: 2170 and G/16155. The former number is his 2/4th number and this actually clears up a little mystery for me. I'd been doing some work recently on Sussex regiment service numbers and noted that a lot of men who had service numbers in the G/15000 range and above, also had five digit G/ numbers as well. The 2/4th Sussex, formed in January 1915, was disbanded in November 1917 and I'm guessing that a lot of men were transferred to New Army battalions from it. When they were transferred they were given another number. There's loads more work to do on this and more patterns in numbering will emerge but I'm reasonably confident on this.
As far as Thomas Clarkson is concerned, his case is a lot simpler. I had his number, 3078, and his battalion, the 13th, but I hadn't twigged that he was an original South Downs volunteer. It's logical of course. Working through the SD/ prefix numbers, patterns emerge for recruitments to the 11th, then the 12th and finally the 13th. Looking at the spread of surnames around 3078 they all begin with C and so Thomas Clarkson fits nicely into the sequence even though the National Archives omits the SD/ prefix from his number.